I had to tension mine about 6 months after purchase, and after a 4-day tour on which neither I nor the saddle ever quite dried out. (In spite of fenders and a seat cover.)
On mine (a B-17 purchased about 18 months ago), you use an open-end wrench. I was surprised to find that they still use 5/16 BSC (a peculiarly British fastener size) -- 13 mm is too small and 14 mm is sloppy.